Constant temperature oven



m W w 3 m n v 0a r 5. m am vs 5 1mm r 2 7 J 2 a w h s C b F i Q. 1

Fig. 2.

C. F. BALDWIN CONSTANT TEMPERATURE OVEN Filed July 30, 1948 June/12, 1951 Patented June 12, 1951 UNITED STATES PATENT OFFICE CONSTANT TEMPERATURE OVEN New York Application July 30, 1948, Serial No. 41,613

6Claims. 1 My invention relates to temperature conditioning apparatus and more particularly to apparatus for maintaining a device at a constant temperature.

Certain devices, whose operating characteristics change upon change in temperature, are desirably maintained at a constant temperature for better operation. It is an object of my invention to provide new and improved means to maintain the temperature of a device within very narrow limits even when ambient temperature changes over wide limits.

It is a further object of my invention to provide an inexpensive, sturdy and compact piezoelectric crystal housing which will maintain the vibratile element at a constant temperature with a high degree of constancy.

My present invention is especially suited to maintain electric circuit elements, such as piezoelectric vibrators, at highly constant temperatures under adverse conditions and over long periods of time.

With certain classes of piezoelectric crystals, particularly those for oscillation in a face shear mode, as for instance, quartz crystals having a CT, D1, or GT cut, it is not practicable to clamp the vibratory plate at its periphery as in thickness shear mode plates, but it has proved much more desirable to mount the plate at a nodal point or zone in the manner shown, for instance, in U. S. Patent No. 2,371,613, Irvin E. Fair, with as little physical supporting area in contact with the plate as possible. While entirely satisfactory results may be obtained by apparatus in accord with my prior U. S. Patents Nos. 2,301,007 and 2,301,008, for peripherally clamped crystals, clamp mounting is not desirable for face shear mode crystals and the present invention provides apparatus more suitable for crystals wherein the limited area of contact between the crystal and its supporting members does not permit satisfactory temperature control of the crystal by conduction through the support. These crystals may be mounted between relatively sharp pins at the nodal zone or by thin rods soldered to the electrodes at the nodal zone.

An additional object of my invention, accordingly, is to provide a small, inexpensive piezoelectric crystal holder capable of maintaining a crystal element at highly constant temperature without relying on thermal conduction to the element.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a partially cut-away side view of apparatus embodying my invention, and Fig. 2 is a sectional view of the apparatus taken along plane 2-2 of Fig. 1.

In Fig. 1 an hermetically sealed container l is welded or soldered to a base 2 provided with connecting terminals or pins 3 all in much the same manner as in conventional metal walled electron discharge devices, and conveniently of approximately the size and shape of a type 6L6 receiving tube. Supported from an upper portion 43 of the base member on insulating spacers 5 is a disk ii forming an internal base member. The disk is of a heat conducting material such as brass. A cap I of brass, steel or other heat conducting material is peripherally bolted around member 6 to form therewith an inner housing which completely encloses the piezoelectric vibrator 8, a mica bafiie 9 and an electric resistance heater [0. The resistance heater comprises a coil of resistance wire, wound about a heat conducting spool ll preferably of brass. The lower portion of the spool may be of reduced diameter and this portion may extend into a suitable opening in or through disk 6 in such manner that heat is readily conducted from the spool to a predetermined central portion of disk member 6, and radially from this portion through member 6 to the inner cap member 1. The base of spool ll may extend through disk 6 and form a central portion of the lower surface thereof, or the base of the spool may extend only partially through the disk and the opening may then extend less than all the way therethrough.

Mounted below disk 6 is a bimetallic thermostat arm l2 supported by a bolt and nut i3 and an insulating washer or bushing M. The thermostat arm receives heat primarily by radiation from the lower surface of the central portion of disk 6, and when it is heated higher than a predetermined temperature, breaks the circuit to heating coil It). An adjustable contact member I5 is arranged for cooperation with the contact portion of arm I2 providin temperature adjusting means. This contact member is conveniently grounded to disk member 6, and the disk is connected through a conductor 16 to one of the external contact pins 3. One end of the heating winding is connected to another of pins 3 through a conductor not visible in Fig. 1 which extends through an insulating bushing in disk 6, and the other end of the winding is connected to insulated thermostat arm I2 through its mounting bolt. Connections to the heater are thus completed when the thermostat is closed from one of the pins 3, through conductor is to disk 6 and adjustable contact member 55, and thence to thermostat arm l2 and the thermostat mounting bolt to one end of the winding, and from the other end of the winding through the other above-mentioned insulated conductor to the above-mentioned other one of pins 3.

Referring now to Fig. 2, in which certain deand the other to a terminal lug l8 mounted under the head of the thermostat arm mounting bolt.

The mounting and connection arrangement for piezoelectric vibrator 8 is best understood by reference to both figures. The vibrator as shown takes the form of a thin plate and may be of quartz. Electrodes for the plate comprise films of metal chemically deposited, sputtered, evaporated or electroplated on the two opposed major faces, or layers of foil suitably attached thereto, as by cementing, and connections are provided to the electrodes by soldering short metal wires or thin rods 59 and 25 to the electrodes at a nodal zone of the plate. The nodal zone of the plate in the drawings is taken to be the center of the plate though it may be elsewhere. The rods i9, 28 extend perpendicularly outward from opposite faces and serve to support the plate 8 as well as to provide electrical connections to the respective electrodes. Conductors 2| and 22 from two re spective additional pins 3 extend upwardly through insulating bushings in inner base memher and protrude above the inner base to form bosses to which are connected supporting conductive wire members 23 and 2d respectively. The wire members 23 and 24 are bent to form horizontal supporting platforms on which the rods 19 and 23 rest to support the crystal plate 3 and to which the rods are respectively soldered or welded. Electrical connections are thus completed from each of the respective crystal electrodes to a corresponding external terminal pin 3. It will be understood that only a very small amount of heat may be conducted to the crystal by rods i9, 26'' and supporting members 23, 2 3. Since the rods are of very small cross section they offer high resistance tothermal conduction although they are of copper or other low electrical resistance material.

The parts shown in Fig. 2 are numbered to correspond to the numbering of Fig. 1. In Fig. 2 the mica baffle 9 is shown arranged below the crystal plate 8, and mounting bolts are indicated for securing inner cap member I peripherally around inner base 6. ternally threaded recesses in disk 6 such as the recess 2s shown in Fig. 1.

In operation, heat generated by winding ill flows into spool H which is in good heat conducting relation with disk 5. The disk and spool, being of brass, or similar material, and of considerable thickness and size, form a substantial heat reservoir. Because of the intimate contact between disk 6 and inner cap l, and because the cap is also of a material which conducts heat readily, the cap l is maintained at nearly as high a temperature as the disk and spool, but, because of the particular arrangement, the temperature. of the cap is maintained more constant than the temperature of the spool and of the center of the disk. The thermostatic arm i2 is arranged for actuation primarily in response to the temperature of the lower surface of the central portion of disk 8 adjacent the base 01"- spool IE, but if the external metal shell I is cooler due to low external ambient temperature the thermostat loses heat more rapidly and tends to maintain the center of disk 8 at a somewhat higher These bolts cooperate with in-,

temperature. This effect is desirable since inner cap 1 loses heat by radiation to the outer shell I and by convection in the air or gaseous atmosphere within the outer shell. This loss of heat by the inner shell is increased by lower ambient temperatures. The thermostat in conjunction .with the heater coil it thus maintains a substantially constant temperature of cap I regard,- less of ambient temperature variations within wide limits.

It may be considered that a heat bridge is established by the arrangement shown. The heat source It) provides, by conduction through spool ll, heat to a predetermined portion of the inner housing, this portion being specifically the'central portion of the circular disk e. Heat flows in one leg of the bridge from this portion radially through the disk and upwardly through inner housing member I. Heat is lost through the other leg in this branch of the bridge from the outer surface-of the housing to the inner surface of the outer housing member i, and thence to the surrounding medium. Heat is also transferred, primarily by radiation, in one leg of the other branch of the bridge from the aforementioned portion of the inner housing to the thermostatic element 52 and is lost through the second leg of this branch from the thermostat to the outer housing. The inner housing and thermostat are arranged, accordingly, in separate branches of the bridge, the two branches being interposed in parallel between the predetermined portion of the inner housing and the outer housing or atmosphere. The thermostat responds rapidly to changes in temperature of the center of disk 6, and the temperature of the inner housing is maintained at a highly constant temperature, the temperature of the center of disk 6 varying between wider limits than that of cap member 1. The limits of variation of the center of the disk depend, of course, primarily upon the sensitivity of the thermostat.

portance. Thereis convection from the heater which tends to heat the crystal element 8, but the convection currents involved are baffied by banle 9 to prevent rapid increase in temperature of the crystal while the heat source is hot. The convection currents have a greater tendency to heat the inner housing directly. These effects tend to compensate for ambient temperature changes in that a drop in ambient temperature of the external medium will cause more rapid cooling of thermostat 12, which will call for 7 heat, by being in a closed position, for a greater percentage of the time. With lower ambient temperatures the heat loss from the inner housing to the outer housing and to the external medium is increased. Accordingly, not enough heat would reach the upperpart oi the inner housing to maintain the crystal temperature constant. The necessary additional heat, however, is provided to thecrystal and to the housing by the additional convection heat flow from the hotter heat source. There will be some radiation'to the crystal and to the inner housing even with the baflle in place, and the bafile itself though of mica, will be heated and will radiate heat.

These radiation effects, like the convection er inner housing. With higher ambient tempera The above explanation omits several'effects of secondary imtures, the reverse occurs since the convection and radiation heating of the crystal are reduced.

The loss from the inner to the outer housing will be primarily by radiation and convection since the insulating support members 5 and connectors such as IE, 2i and 22 offer relatively high resistance to heat flow. The heat transfer from the outer housing member to the external medium, which will be the surrounding air or at mosphere in most instances, may be by radiation, convection and conduction, any one of which may be relatively very large or small, without substantially affecting the operation of the unit. It will be understood that the temperature of the cap i must be maintained at a temperature higher than ambient ii constant temperature of the crystal is to be maintained, though the surrounding mediuro. establishing the ambient temperature may be hotter or colder than free air.

While I have shown only a certain preferred embodiment or" my invention by way of illus-- tration, many modifications will occur to those skilled in the art, and I, therefore, wish to have it understood that I intend, in the appended claims, to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for maintainin constant the temperature of an object comprising a closed, heat-conductive housing, means heat insulated from said housing for supporting said object within said housing, a source of heat within said housing, a bafiie interposed between said source and said object Within said housing arranged to prevent direct heat radiation and direct convection from said source to said object whereby said object is maintained at a temperature determined primarily by radiation and convection with the inner wall of said housing, means for conducting heat from said source to a predetermined portion of said housing, and thermal responsive means adjacent said portion for controlling said source in accord primarily with the temperature of said portion.

2. Apparatus for maintaining constant the temperature of an object comprising an outer closed housing of substantial heat storage capacity, and an assembly within said outer housing comprising an inner, closed, heat-conductive housing, means heat insulated from said inner housing for mounting said object within said inner housing, a source of heat within said inner housing, a baiile interposed between said source and said object within said inner housing arranged to prevent direct radiation and direct convection of heat from said source to said object, whereby said object is maintained at a temperature determined primarily by radiation and convection with the inner surface of said inner housing, means for conducting heat from said source to a predetermined portion of said inner housing, and thermal responsive means adjacent said portion for controlling said source in accord primarily with the temperature of said portion, said outer housing forming a heat reservoir interposed between said assembly and the external medium whereby the effect on said inner housing of rapid changes in ambient temperature are reduced.

3. In combination, an object whose temperature is to be controlled, a heat conductive container for said object, means supporting said object within said container, said means having high resistance to heat conduction, a concentrated source of heat within said container spaced from said object, means for conducting heat from said source to a predetermined portion of said container, temperature responsive control means responsive primarily to the temperature of said portion for controlling the heat supplied by said source, said object being arranged for radiant heat interchange with an inner wall of said container and convection heat interchange with said wall and said heater through the atmosphere within said container, and baffle means interposed between said source and said object to reduce heat interchange by convection or radiation directly from said source to said object.

Temperature conditioning apparatus for a face shear mode vibratory piezoelectric plate comprising a closed container, a concentrate source of heat within said container, said container being arranged to lose heat to the surrounding atmos phere in accord with temperature differential between said casing and the ambient tempera ture, means for conducting heat from said source to a predetermined portion of the bottom wall of said container, said container being of heat conducting material, said plate being arranged within said container in spaced relation with said source and out of thermally conductive contact with the walls of said container, means interposed between said source and said plate for shielding said plate from direct convection and radiation from said source, and thermal responsive control means for said source arranged for actuation in response to the temperature of a portion of said container.

5. Apparatus for maintaining constant the temperature of a nodal zone mounted piezoelectric vibrator comprising a heat conductive base member, means heat insulated from and supported by said base for mounting said vibrator, a heat conductive cap cooperating with said member to form an enclosed housing for said vibrator, said vibrator being positioned out of physical contact with said housing, a heat source within said housing in heat conductive relation with said cap, a battle interposed between said source and said vibrator to prevent radiant heat interchange therebetween, and means responsive to the temperature of said housing for controlling said source.

6. Temperature conditioning apparatus for an object comprising a closed, heat conductive container, means heat insulated from said container for supporting said object within said container, a heat source within said container spaced from said object, means for conducting heat from Said source to said container, a substantially nonheat conductive baffle interposed between said source and object for limiting heat transfer by direct radiation or convection from said source to said object while permitting heat transfer by radiation or convection from said container to said object.

CHARLES F. BALDWIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,873,838 Gebhard Aug. 23, 1932 2,131,826 Thomas Oct. 4, 1938 2,157,703 Hovgaard et a1. May 9, 1939 2,203,545 Peterson June 4, 1940 2,273,711 Klein Feb. 17, 1942 

